The switching power control is a conventional technology used in power converter to control and regulate the output power. Referring to FIG. 1, a circuit schematic of a current mode power converter circuit is shown. A control circuit 20 generates a switching signal SW at an output terminal OUT to regulate the output of the power converter in response to a feedback signal VFB. The switching signal SW drives a power transistor 12 for switching a transformer 10. In general, the feedback signal VFB is obtained at a feedback terminal FB of the control circuit 20 by detecting the output voltage VO of the power converter through an optical-coupler or a feedback circuit including an auxiliary winding (Figure not shown). The voltage of the auxiliary winding is related to the output voltage VO of the power converter. Therefore the feedback signal VFB is generated in response to the output voltage VO. The primary winding NP of the transformer 10 is coupled to receive an input voltage VIN. The secondary winding NS is used to output the output voltage VO.
A rectifier 14 is coupled to the secondary winding NS. A filter capacitor 16 is coupled to the rectifier 14 and the secondary winding NS. A resistor RS is connected serially with the power transistor 12 to generate a current signal VI in response to a switching current IP of the transformer 10. The current signal VI indicates the switching current IP. The current signal VI is coupled to a current-sense terminal VI of the control circuit 20 to develop a current loop for the current mode control.
FIG. 2 shows a circuit diagram of the traditional control circuit 20. The control circuit 20 includes an oscillation circuit 25 and a switching circuit 30. The switching circuit 30 includes a first comparator 32, a second comparator 33, a flip-flip 36 and two AND gates 38, 39 to generate the switching signal SW. The oscillation circuit 25 generates an oscillation signal IPS and transmits the oscillation signal IPS to a clock input terminal CK of the flip-flip 36 for enabling the switching signal SW. An output terminal Q of the flip-flop 36 is connected to an input terminal of the AND gate 39. Another input terminal of the AND gate 39 is coupled to receive the oscillation signal IPS. The first comparator 32 is coupled to receive the current signal VI and a maximum threshold VLIMIT to compare the current signal VI with the maximum threshold VLIMIT. The second comparator 33 is coupled to receive the current signal VI and the feedback signal VFB. The current signal VI is utilized to compare with the feedback signal VFB by the second comparator 33 for regulating the output of the power converter. If the current signal VI is greater than the maximum threshold VLIMIT, the control circuit 20 will disable the switching signal SW and restrict the maximum output power through the AND gate 38. The function of output power limit is generally used for overload and short circuit protections. The operation of the feedback loop and the current limit is shown in FIG. 3. The switching current IP indicates the current signal VI. When the current signal VI is lower than the maximum threshold VLIMIT, the switching signal SW is controlled by the feedback signal VFB. The switching signal SW is restricted by the maximum threshold VLIMIT once the current signal VI is higher than the maximum threshold VLIMIT. In recent development, many technologies have been proposed to optimize the control and protection functions of the current mode power converter. Among them, the slope compensation is described in “Adaptive slope compensator for current mode power converters” by Yang, U.S. Pat. No. 5,903,452. For the output power limit, two compensation schemes are disclosed, such as “PWM controller for controlling output power limit of a power supply” by Yang et al., U.S. Pat. No. 6,611,439; “PWM controller having a saw limiter for output power limit without sensing input voltage” by Yang et al., U.S. Pat. No. 6,674,656. However, the disadvantage of these prior arts is the loose accuracy of the compensation. The slope compensation will affect the compensation of the protection function. This shortcoming is the main object of the present invention to overcome. Besides, a simpler scheme of compensation circuits is required to reduce the cost of the power converter.